1. Field of the Invention
The present invention relates to a thermal flowmeter including at least one resistor element that uses temperature dependence of its electrical resistance to determine a flow rate or velocity of a fluid flowing through a passage. The present invention improves such a flowmeter so as to make the flowmeter more durable under thermal stresses.
2. Discussion of the Related Art
A flowmeter measures flow rate, velocity, or temperature of a fluid, and the fluid may be an intake air introduced into an internal combustion engine. One type of the flowmeter is a thermal flowmeter that includes resistor elements whose electrical resistance varies with an ambient temperature. This thermal flowmeter is disclosed, for example, in laid-open Publication Nos. 55-43447, 56-77716, and 60-91211 of unexamined Japanese Patent Application. U.S. Pat. Nos. 5,224,378 and 5,144,279 also discloses such a thermal flowmeter.
In FIG. 7, the resistor element 70 includes: an electrically insulating substrate 72 having a tubular shape; a metallic resistor 78 disposed around an outer peripheral surface of the substrate 72; a glass coat 79 covering the metallic resistor; and a pair of lead wires 74, 74 fixed to both ends of the electrically insulating substrate 72, the pair of lead wires 74, 74 being electrically connected to the metallic resistor 78. The metallic resistor 78 varies its electrical resistance with an ambient temperature. The metallic resistor may be a metallic film including platinum, etc., and the metallic film may have a spiral pattern. Alternatively, the metallic resistor may be a fine wire winding around the substrate. In either type of the metallic resistor, the glass coat covers the whole substrate and the metallic resistor.
In a thermal flowmeter of FIG. 5, a wall 53 defines a fluid passage 55. A pair of supports 52, 52, which are electrically conductive, protrude from portions, which are electrically insulating, of the wall 53. The pair of supports support the resistor element 70 so that the resistor element is arranged in the fluid passage 55. A pair of lead wires 74, 74 are secured to the pair of respective supports 52, 52 by welding such as spot welding and laser welding.
In the thermal flowmeter of FIG. 5, each support 52 has two ends, and one of the two ends is inserted into the wall 53. In contrast, in the thermal flowmeter of FIG. 6, both of the two ends are inserted into the wall 63.
When the thermal flowmeter detect a parameter, for example, a flow rate of an intake air introduced into the internal combustion engine of an automobile, the thermal flowmeter is subject to temperatures ranging from -40.degree. C. to 110.degree. C., and the thermal flowmeter may undergo a temperature change. The wall 53 thermally expands and contracts according to the ambient temperature. The pair of supports 52, 52 are generally supported by an electrical insulator, which may be made of a synthetic resin having a large expansion coefficient so that the thermal expansion of the synthetic resin produces thermal stresses on the resistor element 70 along its axial direction. As a result, when the thermal flowmeter undergoes a large temperature change or repeated temperature changes, thermal stresses sometimes rupture lead wires 74, 74 of the resistor element 70 and the welding portions 56, 56 that secure lead wires 74, 74 to the supports 52, 52.
The laid-open Publication No. 62-95127 of unexamined Japanese Utility Model Application discloses a resistor element including a pair of curled lead wires so as to buffer stresses on the lead wires by the curled portion. However, it is not easy to work lead wires into the curled shape. Moreover, some curled shapes might make stresses concentrated on the curled portions.